Blake schrieb:
> Dear MathGroup:
>
> I have been blithely using blank sequences in pattern matching for some
> time. In persuit of a bug in a package of mine, I was quite alarmed to
> find that I don't really understand how to use the blank sequence, as
> expressed in the following simplified example:
>
> In[1]:=Replace[a*b*c,Times[mysequence__]:>{mysequence}]
>
> Out[1]={a b c}
>
> I expected Out[1]={a,b,c}, from a naieve reading of the full form of
> a*b*c
>
> In[2]:=FullForm[a*b*c]
>
> Out[2]//FullForm=Times[a,b,c]
>
> Will someone PLEASE tell me why In[1] does not yield the results I
> expected? (I can readily use a work-around, what I am concerned with is
> a correct understanding of pattern matching).
The reason is Times[mysequence__] is immediately stripped to
mysequence__ . Its only one factor present.
Make it a habit to use HoldPattern in order to prevent unrequested
evaluation of the left hand pattern in Rule
In[1]:=Replace[a*b*c,HoldPattern[Times[mysequence__]]:>{mysequence}]
Out[1]={a, b, c}
An unambigous way to exclude pattern evaluation is use of characteristic
Head on the left and head replacement using Apply on the right side in Rule
Replace[a*b*c, expr_Times :> List@@expr]
--
Roland Franzius